Cerebral glucose utilization is reduced in second test session

Cerebral glucose utilization was higher during the first positron emission tomography (PET) session than during the second session, as assayed using the PET [18F]fluorodeoxyglucose method in male human volunteers. This difference was due largely to data from subjects with low-trait anxiety, since subjects with high anxiety showed similar metabolism in both PET sessions. High-anxiety subjects showed greater right/ left ratios of cerebral metabolism than low-anxiety subjects, particularly during the second PET session. These findings suggest that the level of anxiety may be an important variable to consider in PET studies using multiple sessions.

Quantification of neuroreceptors in the living human brain: IV. Effect of aging and elevations of D2-like receptors in schizophrenia and bipolar illness

In a previous study of 10 drug-naive schizophrenic patients, the density of D2 dopamine receptors was found to be elevated in the caudate nucleus. The study raised questions about the influence of the age of the patients, the relationship of receptor density to psychosis, and the accuracy of the method used to obtain this evidence. Using positron emission tomography and constrained analysis of the brain uptake of the radioligand N-[11C]methyl-spiperone ([11C]NMSP), we tested four questions: Were the assumptions underlying the quantitation valid? Is there an age decline of the density of D2-like dopamine receptors in drug-naive schizophrenia and bipolar illness? If so, is it different from that observed in normal aging? Are D2-like dopamine receptors elevated at any age in either drug-naive schizophrenic or psychotic bipolar illness patients? NMSP and haloperidol partition volumes and plasma protein fractions were not significantly different among patient groups and normal volunteers. The model-derived assay of radioligand metabolites in plasma was confirmed by high-performance liquid chromatography in the patient groups. D2-like dopamine receptors declined with age, and the slope did not differ significantly between the schizophrenic patients, bipolar affective illness patients, and normal controls. Taking the effect of age into account, increases in D2 dopamine receptor density were found in seven psychotic patients with bipolar affective illness compared with seven nonpsychotic patients and 24 control subjects as well as in 22 drug-naive schizophrenic patients compared with the 24 control subjects.

Imaging of delta- and mu-opioid receptors in temporal lobe epilepsy by positron emission tomography

The involvement of opioid neurotransmitter systems in seizure mechanisms is well documented. In previous positron emission tomography (PET) studies in patients with unilateral temporal lobe epilepsy, we have found evidence for differential regulation of the opioid-receptor subtypes. The present study extends our previous observations to delta-opioid receptors by using the delta-receptor-selective antagonist [11C]methylnaltrindole ([11C]MeNTI). Paired measurements of delta- and mu-opioid receptor binding and metabolic activity were performed with PET using [11C]MeNTI and [11C]carfentanil ([11C]CFN) and [18F]fluorodeoxyglucose ([18F]FDG), respectively. Binding of [11C]MeNTI and [11C]CFN increased and [18F]FDG uptake decreased in the temporal cortex (TC) ipsilateral to the focus. Decreases in [18F]FDG uptake were more widespread regionally than were increases in opioid receptors. Increases in the delta- and mu-receptor binding showed different regional patterns. Increases in mu-receptor binding were confined to the middle aspect of the inferior TC, whereas binding of delta receptors increased in the mid-inferior TC and anterior aspect of the middle and superior TC. The increase in delta receptors suggests their anticonvulsant action, as previously shown for the delta-receptor subtype, whereas the different regional pattern of receptor alterations suggest the distinct roles of different opioid-receptor subtypes in seizure phenomena.

Fenfluramine-induced loss of serotonin transporters in baboon brain visualized with PET

The present study sought to determine whether or not Positron Emission Tomography (PET) with the newly developed positron emitting serotonin (5-HT) transporter ligand, (+)[11C]McN-5652, could be used to detect fenfluramine-induced 5-HT neurotoxicity in the brain of living primates (baboons). Six PET imaging studies were performed: three before treatment with fenfluramine (5 mg/kg, s.c., twice daily for 4 days) and three after (18, 45, and 81 days after treatment). The dose of fenfluramine used in this study (5 mg/kg) is known to produce 5-HT neurotoxicity in primates, and to be approximately two times higher than a dose of fenfluramine reported to produce small and inconsistent weight loss in baboons (2 mg/kg). Following fenfluramine treatment, marked lasting reductions in regional brain specific binding of (+)[11C]McN-5652 were found by means of PET. Findings with PET corresponded well with post-mortem neurochemical findings indicative of serotonergic neurotoxicity (lasting depletions of regional brain 5-HT, 5-HIAA, and 5-HT uptake sites). These results suggest that PET imaging with (+)[11C]McN-5652 will be useful for evaluating the 5-HT neurotoxic potential of fenfluramine and related drugs in living humans.

Increased mu opioid receptor binding detected by PET in cocaine-dependent men is associated with cocaine craving

The endogenous opioid system has been recently implicated in the reinforcing actions of cocaine and other addictive drugs. In this study we examined mu opioid receptor binding in ten cocaine-dependent men and seven nonaddicted controls using positron emission tomography and [11C] carfentanil. Mu opioid binding was increased in several brain regions of the cocaine addicts studied 1-4 days after their last use of cocaine. Binding was positively correlated with the severity of cocaine craving experienced at the time. The upregulation of mu opioid receptor binding persisted after 4 weeks of monitored cocaine abstinence. These findings demonstrate for the first time the involvement of the endogenous opioid system in cocaine dependence and cocaine craving in living human subjects.

Imaging of delta opioid receptors in human brain by N1'-([11C]methyl)naltrindole and PET

Recently, we have developed the positron emitting radiotracer N1'-([11C]methyl)naltrindole ([11C]MeNTI) and demonstrated its high selectivity for delta opioid receptors in the mouse brain [Lever et al. (1992) Eur. J. Pharmacol., 216:449-450]. In the present study, we examined the selectivity of [11C]MeNTI for the delta opioid receptor in the human brain, using positron emission tomography (PET). The regional kinetics and distribution as well as the pharmacology confirmed the selectivity of [11C]MeNTI for delta opioid receptor in the human brain. First, the regional kinetics of [11C]MeNTI are in accordance with the density of the delta opioid receptor. Rapid washout in receptor-poor areas and prolonged retention in receptor-rich areas were observed. Second, the regional distribution of [11C]MeNTI correlated well (r = 0.91) with the in vitro distribution of delta opioid sites but not with mu or kappa site densities (r < or = 0.008 or r < or = 0.014, respectively). [11C]MeNTI binding was highest in regions of the neocortex (insular, parietal, frontal, cingulate, and occipital), caudate nucleus, and putamen. Binding was intermediate in the amygdala and lowest in the cerebellum and thalamus. Third, studies using the competitive antagonist naltrexone demonstrated the inhibition of [11C]MeNTI binding. Naltrexone inhibition of [11C]MeNTI binding was most effective in delta receptor-rich regions, and its inhibitory potency correlated well (r = 0.88) with the regional distribution of delta opioid sites. [11C]MeNTI is the first radioligand which selectively labels delta opioid receptors in vivo in the human brain following systemic administration. The availability of [11C]MeNTI will enable a receptor specific analysis of the role of [11C]MeNTI receptors in normal and abnormal human brain.

Peptide T and glucose metabolism in AIDS dementia complex

AIDS dementia complex (ADC) is the most common presenting neurologic manifestation of human immunodeficiency virus (HIV)-1 infection. We report FDG-PET studies in a 39-year-old man who had ADC and completed a 12-wk treatment protocol with 1.2 mg/day of intranasal peptide T, one before and one after 12 wk of treatment with peptide T. Peptide T is an octapeptide under investigation for treatment of ADC patients. Values of rCMRglc were converted to Z scores using the mean and standard deviation of values of rCMRglc in three HIV-seronegative matched controls, each of which was studied twice, at the beginning and end of a 12-wk interval. Thirty-five of 60 regions assayed showed Z scores with absolute values > or = 3 (considered abnormal) in the baseline study. Regions with high absolute values of Z scores were located in subcortical areas and in the limbic system, and to a lesser degree in the frontal, temporal and parietal lobes. Thirty-four of these 35 regions showed remission (decrease in the absolute values of Z scores) after treatment. Only one region showed no improvement in the second study. Three regions with absolute values of Z scores < 3 in the baseline study manifested Z scores with magnitudes > or = 3 in the second study. These preliminary observations suggest that functional neuroimaging techniques provide a useful tool in the evaluation of the response to treatment in ADC patients.

Brain metabolism patterns are sensitive to attentional effort associated with a tone recognition task

Using positron emission tomography with the tracer 18-fluoro-D-deoxyglucose, we assessed regional cerebral glucose utilization patterns (rCMRglu) associated with three performance levels in a forced choice, tone recognition task. Four normal subjects responded with one hand when they heard a high-frequency tone (1500 Hz), and with the other hand when they recognized a low-frequency tone (750 Hz). The EASY (EAS) condition accuracy average was 96%, the INTERMEDIATE level accuracy averaged 89%, and the DIFFICULT (DIF) recognition task accuracy average was 77%. Statistical parametric mapping (SPM94) analysis revealed that the DIF minus EAS contrast is associated with a marked metabolic elevation in the right middle and inferior temporal gyri and the gyrus fusiformis. The EAS minus DIF contrast revealed greater rCMRglu in the right medial geniculate body. Enhanced activity in right temporal lobe structures may reflect a role in auditory memory and "image" visualization. The medial geniculate enhancement may reflect tone frequency assessment.

Dopamine transporters are markedly reduced in Lesch-Nyhan disease in vivo

Dopamine (DA) deficiency has been implicated in Lesch-Nyhan disease (LND), a genetic disorder that is characterized by hyperuricemia, choreoathetosis, dystonia, and compulsive self-injury. To establish that DA deficiency is present in LND, the ligand WIN-35,428, which binds to DA transporters, was used to estimate the density of DA-containing neurons in the caudate and putamen of six patients with classic LND. Comparisons were made with 10 control subjects and 3 patients with Rett syndrome. Three methods were used to quantify the binding of the DA transporter so that its density could be estimated by a single dynamic positron emission tomography study. These approaches included the caudate- or putamen-to-cerebellum ratio of ligand at 80-90 min postinjection, kinetic analysis of the binding potential [Bmax/(Kd x Vd)] using the assumption of equal partition coefficients in the striatum and the cerebellum, and graphical analysis of the binding potential. Depending on the method of analysis, a 50-63% reduction of the binding to DA transporters in the caudate, and a 64-75% reduction in the putamen of the LND patients was observed compared to the normal control group. When LND patients were compared to Rett syndrome patients, similar reductions were found in the caudate (53-61%) and putamen (67-72%) in LND patients. Transporter binding in Rett syndrome patients was not significantly different from the normal controls. Finally, volumetric magnetic resonance imaging studies detected a 30% reduction in the caudate volume of LND patients. To ensure that a reduction in the caudate volume would not confound the results, a rigorous partial volume correction of the caudate time activity curve was performed. This correction resulted in an even greater decrease in the caudate-cerebellar ratio in LND patients when contrasted to controls. To our knowledge, these findings provide the first in vivo documentation of a dopaminergic reduction in LND and illustrate the role of positron emission tomography imaging in investigating neurodevelopmental disorders.

Development of [11C]L-159,884: a radiolabelled, nonpeptide angiotensin II antagonist that is useful for angiotensin II, AT1 receptor imaging

[11C]L-159,884 ([11C] N-[[4'[(2-ethyl-5,7-dimethyl-3H- imidazo[4,5-b]pyridin-3-yl) methyl] [1,1'-biphenyl]-2-yl] sulfonyl]-4-methoxybenzamide) and [11C]L-162,574 ([11C] N-[[4'[2-ethyl-5,7- dimethyl-3H-imidazo[4,5-b] pyridin-3-yl)methyl] [1,1'-biphenyl]-2-yl]sulfonyl]-3- methoxybenzamide), both potent and selective ligands for the AT1 receptor, were prepared by C-11 methylation of the corresponding desmethyl phenolic precursors. The radiotracers were purified by semi-preparative reverse-phase HPLC. Non-decay corrected radiochemical yields were 5 and 3% for L-159,884 and L-162,574 respectively, and the average specific activity was 2979 mCi/mumol at end-of-synthesis (EOS). The average time of synthesis was 18 min.

Assessing neuroreceptor occupancy by continuous infusion of carbon-11 labeled radioligands

In carbon-11 labeled radiotracer studies of neuroreceptors in the brain, a state of practical equilibrium, monitored with a simple dual-probe detector system, was achieved by injection of a bolus of tracer followed by a continuous infusion of the tracer. After investigating several different bolus-to-infusion activity ratios, we observed that a practical equilibrium state could be achieved with a slope of 0.02+/-0.001 cpsxkg/microCi/min for [11C]carfentanil studies in normal human beings. The usefulness of this approach to assess neuroreceptor occupancy was demonstrated in two cases.

In vivo labeling of angiotensin II receptors with a carbon-11-labeled selective nonpeptide antagonist

Angiotensin II (ANG II) initiates a variety of physiological effects by binding to high affinity receptors. Two ANG II receptor subtypes, AT1 and AT2, have recently been identified. This study was undertaken to evaluate [11C]L-159,884, an AT1 subtype selective nonpeptide antagonist, as a potential PET tracer.

METHODS

Carbon-11-L-159,884 was prepared by alkylation of the nor precursor with [11C]methyliodide and was studied for its in vivo binding characteristics, biodistribution and kinetics in mice. The effects of PD-123319, an AT2-selective ANGII antagonist, as well as those of alpha- and beta-adrenergic drugs on [11C]L-159,884 binding were investigated also.

RESULTS

Administration of the AT1 antagonists resulted in dose-dependent inhibition of [11C]L-159,884 binding in the kidneys, the organ with the highest density of AT1 receptors. Inhibition was also observed in the lungs and the heart. Adrenergic drugs did not influence [11C]L-159,884 binding to AT1 receptors. Kinetic studies showed rapid tracer uptake in the liver, kidneys, lungs and heart. Excretion of the radioactivity occurred primarily through the intestinal tract (> 20% in 90 min), with less than 8% excreted through the urine.

CONCLUSION

The results suggest that [11C]L-159,884 binds in vivo to AT1 receptors in mouse kidneys, lungs and heart. This radiotracer appears to be a promising candidate for studying ANG II receptors in vivo by PET.

3 Beta-(p-trimethylsilylphenyl)tropane-2 beta-carboxylic acid methyl ester: a new precursor for the preparation of [123I]RTI-55

Here we report the synthesis of 3 beta-(p-trimethylsilylphenyl)tropane-2 beta-carboxylic acid methyl ester, a new, easily prepared precursor for the synthesis of radioiodinated versions of RTI-55. In contrast to the 3 beta-(p-trimethylstannylated) precursor for radioiodination, the aryl silane is prepared directly by the 1,4 addition of p-trimethylsilylphenylmagnesium iodide to anhydroecgonine methyl ester in 49% yield (Scheme 1). Radioiododesilylation proceeded smoothly (approx. 85% radioincorporation after 15 min at room temperature) using 1 microgram of the novel precursor in trifluoroacetic acid and chloramine-T as oxidant. Isolated radiochemical yields for 125I and 123I radiolabelings were 72 and 48%, respectively, with a calculated specific activity for [123I]RTI-55 of > 2000 mCi/mumol.

Functional sites of neuroleptic drug action in the human brain: PET/FDG studies with and without haloperidol

OBJECTIVE

The functional pathways through which antipsychotic drugs act in the brain to decrease psychosis remain unknown, despite our knowledge that their site of initial action is through blockade of dopamine D2 receptors. The authors sought to define the brain regions that are functionally altered by neuroleptic drugs.

METHODS

Regional cerebral glucose metabolism was studied in 12 subjects with schizophrenia while they were receiving a fixed dose of haloperidol, again 5 days after withdrawal of the drug, and a third time 30 days after withdrawal. Positron emission tomography with an [18F]fluorodeoxyglucose tracer was used in a within-subject design.

RESULTS

The analysis demonstrated a decrease in glucose metabolism in the caudate and putamen 30 days after withdrawal, indicating that haloperidol treatment enhanced glucose utilization in these areas. The thalamus, bilaterally but only in anterior areas, showed the same response to haloperidol. Only in the frontal cortex and in the anterior cingulate had metabolism increased 30 days after withdrawal, indicating that in those two cortical areas haloperidol depressed glucose metabolism. In the 5-day drug free scans, no regions differed significantly from those in the haloperidol condition, despite numerical changes.

CONCLUSIONS

It appears that 5 days of neuroleptic withdrawal are inadequate to escape the effects of neuroleptic drugs on regional cerebral glucose metabolism. The pattern and localization of changes in metabolic activity between the haloperidol condition and the 30-day drug-free condition suggest that haloperidol exerts its primary antidopaminergic action in the basal ganglia. It is proposed that the additional changes in the thalamus and cortex are secondary to this primary site of drug action, mediated through classically described striato-thalamo-cortical pathways.

Positron emission tomography of 5-HT reuptake sites in the human brain with C-11 McN5652 extraction of characteristic images by artificial neural network analysis

The goal of this work was to identify the distribution of serotonin transporters in the human brain with [11C](+)McN5652/PET. Four healthy volunteers were studied. To determine non-specific binding, a PET study was also performed with the inactive enantiomer [11C](-)McN5652 as well as with [11C](+)McN5652 after pretreatment with fluoxetine. For pattern extraction the PET data sets were analyzed by a back-propagation neural network. Two pharmacokinetic patterns and two characteristic images were separated; one representing specific binding, the other representing non-specific binding. The specific binding image showed characteristic distribution of serotonin transporters with [11C](+)McN5652. The pattern images demonstrated an improvement in image quality compared to the original PET images (reduced variance, higher region-to-cerebellum ratio, good correlation with known density of serotonin transporters). The non-specific binding images extracted from [11C](-)McN5652/PET were similar to those of [11C](-)McN5652 and [11C](-)McN5652 with fluoxetine. Thus, PET studies obtained with [11C](+)McN5652 largely represent the regional distribution of the serotonin transporters and the inactive enantiomer [11C](-)McN5652 shows the distribution of its nonspecific binding.

[3H]A-69024: a non-benzazepine ligand for in vitro and in vivo studies of dopamine D1 receptors

[3H]A-69024 has been prepared as a radioligand for studying the dopamine D1 receptor. [3H]A-69024 binds to rat striatal membranes with a KD = 14.3 +/- 3.2 nM (mean +/- SEM; n = 3) and Bmax = 63.5 +/- 12.8 fmol/mg wet tissue (1.8 +/- 0.3 pmol/mg protein). This ligand binds to only one site with a Hill coefficient close to unity. The in vivo biodistribution of [3H]A-69024 showed a high uptake in the striatum (5.9% ID/g) at 5 min followed by clearance. As a measure of specificity, the striatum/cerebellar ratio reached a maximum of 6.7 at 30 min post-injection. Pre-treatment with the D1 antagonist R(+)SCH 23390 (1 mg/kg) reduced this ratio to unity. The dopamine antagonist (+)butaclamol and unlabeled A-69024 inhibited striatal uptake by 70 and 51%, respectively. Spiperone (D2/5-HT2A) and ketanserin (5-HT2A/5-HT2C) at doses of 1 mg/kg had no inhibitory effect on [3H]A-69024 uptake in the striatum; however, increased uptake of [3H]A-69024 by > 30% in the whole brain was observed. The selectivity and affinity of [3H]A-69024 suggests that this non-benzazepine radioligand may be useful for in vitro and in vivo studies of the dopamine D1 receptor.

Positron emission tomography of 5-HT transporter sites in the baboon brain with [11C]McN5652

[11C]McN5652 is a new radioligand specific for 5-hydroxytryptamine (5-HT; serotonin) transporters. In this study we used [11C]McN5652 to image the 5-HT transporter sites in baboon brain by positron emission tomography (PET). Dynamic PET studies were performed in three Papio anubis baboons. The animals were injected intravenously first with 11C-labeled (+)-McN5652([11C](+)McN5652), then with pharmacologically inactive enantiomer 11C-labeled (-)-McN5652 ([11C](-)McN5652); two animals received a third study with [11C](+)McN5652 after pretreatment with the specific 5-HT uptake site inhibitor fluoxetine (5 mg/kg). Initial uptake into the brain was similar for both [11C](+)McN5652 and [11C](-)McN5652. At later times (45-120 min after injection), only [11C](+)McN5652 showed a distribution characteristic for 5-HT uptake sites. In contrast, in studies with [11C](-)McN5652 and in those with [11C](+)McN5652 after 5-HT uptake site blockade with fluoxetine, 11C radioactivity concentrations were significantly lower and the distribution pattern was relatively even. The differences between [11C](+)-and (-)McN5652 were calculated for the time interval 95-125 min postinjection and used to estimate specific binding. Specific binding correlated well (r = 0.95, p < 0.001) with the known density of 5-HT uptake sites in human brain. These results indicate that [11C](+)McN5652 is suitable for PET imaging of 5-HT uptake sites in primate brain.

Selective alkylation of pyrimidyl dianions III: no-carrier-added synthesis of [11C-methyl]-thymidine

A no-carrier-added, high specific activity synthesis of [11C-methyl]-thymidine is reported. Reaction of 3'. 5'-O-bis-(tetrahydropyramyl)-5-bromo-2'-deoxyuridine with n-butylithium produced a diamion which was alkylated with [11C]-methyl iodide, and on subsequent hydrolysis, yielded [IIC-methyl]-thymidine. The labeled compound was isolated from the by-product 2'-deoxymidine by HPLC on a reverse phase C18 semipreparative column with mean radiochemical yield of 18.8% (decay corrected) in 30-35 min and radiochemical purity >99%. This no-carrier-added synthesis can be used to produce [11C-methyl]-thymidine with mean specific activity over 1000 mCi/mumol for positron emission tomography (PET) studies.

Cerebral glucose utilization in polysubstance abuse

Regional cerebral glucose metabolism in subjects with histories of polysubstance abuse was compared to that in control subjects who were drawn from the same community. The substance abuse group showed lower absolute metabolic rates for glucose in lateral occipital gyrus and higher normalized metabolic rates in temporal and frontal areas, including orbitofrontal cortex. It is suggested that some patterns of brain function associated with polysubstance abuse may represent consequences of drug exposure, or they could reflect pre-existing differences that may be relevant to the etiology and maintenance of polysubstance abuse.

Synthesis and PET imaging of the benzodiazepine receptor tracer [N-methyl-11C]iomazenil

The central benzodiazepine receptor tracer [N-methyl-11C]iomazenil (Ro 16-0154) was synthesized by alkylation of the desmethyl precursor noriomazenil with [11C]methyl iodide. The [11C]CH3I (prepared by reduction of [11C]CO2 with LiA1H4 followed by reaction with HI) was reacted with noriomazenil in N,N-dimethylformamide and Bu4N+OH- for 1 min at 80 degrees C and purified by HPLC (C18, 34% CH3CN/H2O 7 mL/min). The product was obtained with synthesis time 35 +/- 5 min (mean +/- SD, n = 7), radiochemical yield (EOB) 36 +/- 16%, radiochemical purity 99 +/- 1%, and specific activity 5100 +/- 2800 mCi/mumol. Absorbed radiation doses were calculated from previously acquired human biodistribution data. The urinary bladder wall received the highest dose (0.099 mGy/MBq) for 4.8 h voiding interval and the effective dose equivalent was 0.015 mSv/MBq. After i.v. injection of [11C]iomazenil in an adult baboon or healthy human volunteer, radioactivity accumulated in the cortex with time-activity curves in agreement with results obtained with [11C]flumazenil PET and [123I]iomazenil SPECT studies. The count rate was sufficient to obtain quantitative images up to 2 h post-injection with a 14 mCi injection. These results suggest that [11C]iomazenil will be a useful agent for measuring benzodiazepine receptors in vivo by positron emission tomography.

In vivo D2 dopamine receptor density in psychotic and nonpsychotic patients with bipolar disorder

BACKGROUND

A prior positron emission tomographic study from The Johns Hopkins University, Baltimore, Md, using N-methylspiperone labeled with carbon 11 reported elevated basal ganglia D2 dopamine receptor density (Bmax) values in neuroleptic-naive schizophrenic patients compared with controls. We have now extended these studies to include patients with bipolar disorder.

METHODS

Patients with bipolar disorder (n = 14) either had never received neuroleptic medication or had been neuroleptic-free for more than 6 months, and they met DSM-III criteria for currently symptomatic affective disorder. Patients with bipolar disorder were compared with matched schizophrenic patients and normal controls. All received two positron emission tomographic scans, the second of which was preceded by oral administration of haloperidol lactate, to permit the calculation of D2 dopamine receptor Bmax.

RESULTS

Diagnostic groups differed in Bmax by analysis of variance (P < .0001); post hoc tests showed higher Bmax values for psychotic patients with bipolar disorder and schizophrenic patients compared with normal controls and for schizophrenic patients and psychotic patients with bipolar disorder compared with nonpsychotic patients with bipolar disorder. Among patients with bipolar disorder, Bmax values correlated significantly with the severity of psychotic symptoms (r = .63) on the Present State Examination but not with the severity of nonpsychotic mood symptoms.

CONCLUSIONS

We conclude that, like schizophrenic patients, patients with psychotic bipolar disorder have elevations of D2 dopamine receptor Bmax values and that such elevations in affective disorder are more closely associated with the presence of psychosis than with mood abnormality. Elevations in dopamine receptor values thus may occur in psychiatric states that are characterized by psychotic symptoms rather than being specific to schizophrenia.

An improved method for the synthesis of radiolabeled McN5652 via thioester precursors

An improved procedure that facilitates routine production and increases the radiochemical yield of [11C]McN5652 (trans-1,2,3,5,6,10b-hexahydro-6-[4-([11C]methylthio)-phenyl]pyrrolo- [2,1-alpha]-isoquinoline) is presented. Specifically, thiol acetate, butyrate, and benzoate derivatives of McN5652 were prepared as the precursors for the [11C]McN5652 synthesis. These thioesters offer greater stability than the previously used thiol precursor (desmethyl McN5652) and enable a single batch of material to be used for multiple radiolabelings. Hydrolysis of the thioester functionality (tetrabutylammonium hydroxide, 10 min) unmasked the free thiol which, without purification, was reacted with [11C]iodomethane in DMF at 40-45 degrees C for 1 min. The average decay-corrected radiochemical yield for [11C]McN5652 was 26% with an average specific activity of 2290 mCi/mumol (end of synthesis). This facile radiolabeling method, utilizing the butyrate thioester of McN5652, was also employed in the preparation of [3H](+)- and (-)McN5652 [trans-1,2,3,5,6S (or 6R),10bR, (or 10bS)-hexahydro-6-[4-([3H]methylthio)phenyl]pyrrolo-[2,1,alpha]- isoquinoline] from [3H]iodomethane.

[11C]A-69024: a potent and selective non-benzazepine radiotracer for in vivo studies of dopamine D1 receptors

[11C]A-69024, (+/-)-1-(2-bromo-4,5-dimethoxybenzyl)-7-hydroxy-6-methoxy-2-[11C]methyl- 1,2,3,4-tetrahydroisoquinoline, is a specific and selective dopamine D1 radiotracer. The in vivo biodistribution of this novel radioligand in mice showed a high uptake in the striatum (6.7% ID/g) at 5 min, followed by clearance with a half-life of 16.1 min. As a measure of specificity, the striatal/cerebellar ratio reached a maximum of 7.4 at 30 min post-injection. Radioactivity in the striatum was reduced to the level of the cerebellum by pre-administration of the D1 antagonist SCH 23390 (1 mg/kg). Pretreatment of mice with spiperone (D2), 7-hydroxydipropylaminotetralin (7-OH-DPAT) (D3), clozapine (D4), ketanserin (5-HT2/5-HT2C), mazindol (monoamine reuptake), prazosin (alpha 1), and haloperidol (D2/sigma) had no inhibitory effect on [11C]A-69024 uptake in the striatum. The dextrorotatory enantiomer of the dopamine antagonist butaclamol inhibited striatal uptake, while the less active isomer (-)-butaclamol did not. [11C]A-69024 binding was inhibited by unlabeled A-69024 in a dose dependent manner (ED50 = 0.3 mg/kg) in the striatum while no change occurred in the cerebellum. [11C]A-69024 warrants further investigation as a PET ligand for examination of central dopamine D1 receptors in humans.

Highly potent indanamine serotonin uptake blockers as radiotracers for imaging serotonin uptake sites

Two highly potent indanamine serotonin (5-HT) uptake blockers, trans-3'-(4'-bromophenyl)-1-indanamine (trans-[11C]DBPI or [11C]Lu 19-056) and its iodo analog, trans-3'(4'-[125I]iodophenyl)-1-indanamine (trans-[125I]DIPI) were evaluated as radiotracers for imaging 5-HT uptake sites in vivo Trans-[11C]DBPI was synthesized by N-methylation of the normethyl precursor with [11C]iodomethane. Trans-[125I]DIPI was synthesized by iododestannylation of the tributyltin precursor with [125I]NaI. Radiochemical yields for the [11C] and [125I] radiotracers were 34 and 40% with specific activities of 4000 and 1800 mCi/mumol, respectively. In vitro, the iodo analog, trans-DIPI, showed an IC50 value of 0.26 nM in inhibition of [3H]paroxetine binding to 5-HT uptake sites in rat cortex. The potency was found to be equivalent to that of paroxetine or McN5652. In vivo, after i.v. injection into mice, both radiotracers showed high uptake in brain (3-4% dose/whole brain at 15 min) and high accumulation into target tissues such as hypothalamus and olfactory tubercles (7-8% dose/g at 60 min). The binding was blocked by pre-injection of 5 mg/kg of peroxetine. While the in vivo distribution agreed with previously reported 5-HT uptake site distribution, the radiotracers showed high uptake in non-target tissues such as cerebellum, resulting in low target-to-non-target ratios (1.5-1.6 at 60 min). Since washout from non-target regions was slower than from target regions, longer-time observation with 125I up to 6 h did not improve the ratios. HPLC analyses of mouse brain homogenates and blocking studies indicated that the high uptake in non-target regions is not the result of metabolism or any interaction of the radiotracers with those tissues via specific binding sites. In spite of low target-to-non-target ratios, target regions with high density of 5-HT uptake sites, such as the raphe nuclei, superior colliculi and substantia nigra, were visualized with trans-[125I]DIPI by ex vivo autoradiography, since the radiotracer showed high specific binding (total mimus nonspecific binding).